Since a great magnetoresistance (GMR) was discovered in a ferromagnetic thin film, its application has been suggested to various fields. A typical application thereof is a magnetic random access memory (MRAM), which is a solid-state memory that can re-write and read recorded data many times by using magnetization direction of ferromagnets.
A unit cell in an MRAM has a multilayered ferromagnet-laminated structure. The relative alignment (being parallel or antiparallel) of the magnetization of the multilayers that consists the memory cell corresponds to “0” or “1” in binary data for recording. An MRAM has theoretically zero power consumption for maintaining data, and it corresponds to a nonvolatile memory that can maintain the data even when the power is off.
The MRAMs utilize magnetoresistance devices which typically have a tri-layered structure of free-ferromagnet/spacer/fixed-ferromagnet. The spacer can be either an insulator, as in a magnetic tunnel junction (MTJ), or a non-magnetic metal, as in a spin valve. In those devices, the magnetic sensitivity and device's performance can be improved by adding a pinning layer, such as an antiferromagnet (AFM) or a synthetic antiferromagnet (SAF), adjacent to the fixed-ferromagnet to magnetically pin it in one direction.
Conventionally, data recording (writing process) is performed by reversing the magnetization direction of the free-ferromagnet through electromagnetic field generated by a current flow in a bit line and a word line, perpendicular to each other, and both parallel to the ferromagnet plane. This field-induced magnetization switching method exhibits an inherent disadvantage that the switching field, therefore the required current, is drastically increased with decreasing size of the cells. Furthermore, crosstalk between the adjoining lines and cells becomes serious when the cell-distance is small. These put a limitation on increasing the density of the magnetic memory device. In contrast, the current-induced magnetization switching (CIMS), a novel attractive alternative, allows to solve those problems. In the CIMS, a current flows perpendicular to plane of a ferromagnet can reverse its magnetization due to spin torque transferred from the spin-polarized conduction electrons to the magnet. The free-ferromagnet can be switched in parallel or antiparallel direction with the fixed ferromagnet when the current flows from the free to the fixed layers, or from the fixed to the free, respectively. In this mechanism, switching occurs when the current density reaches a critical value which is dependent on the spin-transfer efficiency of the current in that device. Therefore, in the same device structure, the smaller the size is, the lower switching current is required. However, there are two critical issues to be solved so that CIMS can be applied in real devices: the switching current density must be lowered and the output signal of the device must be increased.